An example of an image heating apparatus that uses an electromagnetic induction heating method is disclosed in Unexamined Japanese Patent Publication No. HEI 10-74009.
FIG. 1 is an oblique drawing of an image heating apparatus disclosed in Unexamined Japanese Patent Publication No. HEI 10-74009, showing an example of an image heating apparatus that uses a magnetic flux absorption member that absorbs magnetic flux.
In FIG. 1, reference number 1 indicates a metal sleeve that produces heat by means of induction heating. Metal sleeve 1 is mounted on, and supported in a rotatable fashion by, the outer periphery of a cylindrical guide 7. Reference number 2 indicates a pressure roller that exerts pressure on metal sleeve 1. An unfixed toner image formed on recording paper 8 is heat-fixed when recording paper 8 passes through the nip area (pressure area) between metal sleeve 1 and pressure roller 2. Reference number 4 indicates an exciting coil that is installed inside of guide 7 and generates a high-frequency magnetic field, and reference numbers 6a and 6b indicate magnetic flux absorption members that are located on the outside of metal sleeve 1 and absorb magnetic flux.
Recording paper 8 bearing an unfixed toner image is transported to the nip area in the direction indicated by the arrow S. A fixed toner image is then formed on recording paper 8 by the heat of metal sleeve 1 and the pressure between metal sleeve 1 and pressure roller 2. In this example, recording paper 8 is transported with a reference position at its right-hand side in FIG. 8, and if the paper width varies, the left-hand side in FIG. 8 is a paper non-passage area.
As shown in FIG. 1, magnetic flux absorption member 6b on the left-hand side is configured so as to be capable of parallel movement in the axial direction along a rail 5 through rotation of a motor 3.
When wide recording paper 8 is passed through, magnetic flux absorption member 6b is placed in a position facing metal sleeve 1 without the intermediation of magnetic flux absorption member 6a. 
On the other hand, when narrow recording paper 8 is passed through, magnetic flux absorption member 6b is moved to the rear of magnetic flux absorption member 6a as shown in FIG. 2. By this means, magnetic flux reaching metal sleeve 1 from exciting coil 4 in the paper non-passage area is reduced. Therefore, the calorific value of the ends of metal sleeve 1 is suppressed.
Thus, the temperature rise in the paper non-passage area of metal sleeve 1 is reduced according to the width of recording paper 8.
However, with the image heating apparatus shown in FIG. 1, in order to perform parallel movement of magnetic flux absorption member 6b, the distance between movable magnetic flux absorption member 6b and metal sleeve 1 and the distance between magnetic flux absorption member 6a and metal sleeve 1 are different, as shown in FIG. 2. Consequently, a difference tends to occur between the calorific values of the part where movable magnetic flux absorption member 6b is facing metal sleeve 1 and the part where magnetic flux absorption member 6a is facing metal sleeve 1. Therefore, it is not easy to heat the entire width of metal sleeve 1 uniformly.
FIG. 3 is an oblique drawing of another image heating apparatus disclosed in Unexamined Japanese Patent Publication No. HEI 10-74009, showing an example of an image heating apparatus that uses a magnetic flux shielding plate as a means of reducing magnetic flux acting upon metal sleeve 1.
In the conventional image heating apparatus shown in FIG. 3, a magnetic flux shielding plate 9 is positioned so as to be in line with the inner surface of a holder 10 between metal sleeve 1 and exciting coil 4. Then, when narrow recording paper 8 is passed through, magnetic flux shielding plate 9 is moved to a position where it covers exciting coil 4 over an axial direction range equivalent to the paper non-passage area of metal sleeve 1, and when wide recording paper 8 is passed through, magnetic flux shielding plate 9 is retracted to the outer edge of the paper passage width of metal sleeve 1. Thus, the entire width of metal sleeve 1 is heated uniformly when wide recording paper 8 is passed through.
However, in the image heating apparatus shown in FIG. 3, since magnetic flux shielding plate 9 is installed so as to be in line with the inner surface of holder 10 between metal sleeve 1 and exciting coil 4, magnetic flux masking shield 9 must be made thin. When magnetic flux shielding plate 9 is made thin, heat production due to induction heating increases. Moreover, as holder 10 is generally of a plastic material with low thermal conductivity, there is little heat dissipation from magnetic flux shielding plate 9 to holder 10. There is consequently a possibility that magnetic flux shielding plate 9 will continue to rise in temperature.
Furthermore, a problem with the image heating apparatus shown in FIG. 1 is that a mechanism is necessary to perform parallel movement of magnetic flux absorption member 6b, making the configuration of the overall apparatus complex and large.